1. Field of the Invention
This invention relates to a spread spectrum communication system containing a modulator and a demodulator.
2. Description of the Prior Art
Spectrum spread systems are widely used for a local communicating means, such as wireless local area networks (LAN), communications with a weak electric wave, and the like.
Spread spectrum communication systems have a variety of systems such as a direct spread (DS) system, a frequency hopping (FH) system, or a combined type of the above two systems.
In the DS system, when an information signal is a(t), an output of a primary modulator is b(t), and a spread code is c(t), a spread-modulated signal s(t) is defined by EQU s(t)=b(t).times.c(t).
If the data rate of the information signal a(t) is Ra (bit/sec), the symbol rate of the primarily modulated signal b(t) is Rb (symbol/sec), and the chip rate of the spread-modulated signal s(t) is Rc (chip/sec), a bandwidth W(Hz) of the spread-modulated signal s(t) after being spectrum-spread, a spectral spread ratio K and a multilevel index m (an index representing phase shifting quantity) of the primary modulator (here, m=n) are defined respectively as follows; EQU W=2Rc EQU K=Rc/Rb EQU Ra=mRb.
Accordingly, the data rate Ra of the information signal is shown by EQU Ra=mW/2K.
From the above equation, it is found that the larger a value of m is set, the more the data rate of the information signal (Ra) is accelerated.
Primary modulation used in the conventional DS spread spectrum system has usually employed Quaternary Phase-Shift Keying (hereinafter, referred to as QPSK) (m=2), and to increase the multilevel index, 8PSK (Phase-Shift Keying) (m=3) or 16PSK (m=4) has been developed, for example.
Actually, the data rate of the 8PSK (or 16PSK is 1.5 to 2 times faster than that of QPSK, but the phase between signals in the 8PSK and 16PSK are .pi./8 and .pi./16 respectively, while the phase between signals in QPSK shows .pi./4. Such decrease in the phase naturally brings about shortening the interval of signals, causing deterioration of the bit error rate in proportion to the multilevel index m under the same signal-to-noise condition.
Therefore, in case of increasing multilevel index in PSK, it is absolutely necessary to employ error correction coding, etc., which will decrease data rate of the information signal by the duration spent for its encoding rate.